Thursday, September 18, 2014

September Precipitation Days

After the tremendous deluge on September 1st and 2nd, no measurable precipitation has been observed at the Fairbanks International Airport. Is this unusual? Since 1915, the average amount of precipitation between September 3rd and September 17th is 0.49". This is only the 4th year (out of 99) with no precipitation during those dates. The other years were 1963, 1974, and 2006. Also, there are normally 4.3 days with measurable precipitation during these 15 days; but of course none this year.

For the entire month of September, the long-term average number of days with measurable precipitation at the Fairbanks International Airport is 9.8. When days with Trace values are included, the number jumps up to 16.0 days. The 9.8 days with measurable precipitation is actually on the low end of other readings around Fairbanks. Figures 1 shows the normal (average) number of days with measurable precipitation in the Fairbanks area for all stations with at least 10 complete years of precipitation data since 1950. Figure 2 shows all days with at least a Trace of precipitation.

Figure 1. Long-term average number of days with measurable precipitation in the greater Fairbanks area for all stations with at least 10 complete years of summer precipitation data since 1950.

Figure 2. Long-term average number of days with any precipitation (Trace or measurable) in the greater Fairbanks area for all stations with at least 10 complete years of summer precipitation data since 1950.

Readers of this blog probably know that September is past the peak of the wet season for Fairbanks. July is typically the month that observes the most rainfall and the most days with measurable precipitation. Figure 3 show which portions of Alaska are wettest during which month. Most areas south of the Alaska Range are wettest in September through December.

Figure 3. The wettest month of the year across Alaska based on NCDC normals.

At the statewide level, we see that a significant gradient exists with regard to the number of days with measurable precipitation during the month of September from northeast to southwest (see Figure 4). Approximately 1/3 of days see measurable precipitation northeast of a line from Glenallen to Wainwright. Southwest of that line, significantly more days with measurable precipitation are observed.

Figure 4. Long-term average number of days with measurable precipitation in the state of Alaska area for all stations with at least 10 complete years of summer precipitation data since 1950.

What is really interesting is to see the number of days with any precipitation (Trace values included) at the statewide level (see Figure 5). Note the very large number of Trace+ days along the North Slope and the west coast. The portion of the state that sees half or fewer days with a Trace or more of precipitation is now much smaller – with a minimum around Tok.

Figure 5. Long-term average number of days with any precipitation (Trace or measurable) in the state of Alaska for all stations with at least 10 complete years of summer precipitation data since 1950.